The present invention pertains to a sewing machine with at least one feed device a stitch regulating device, an adjusting device for the stitch regulating device, a pulse generator detecting the speed and the angular position of a machine shaft, and a computer, which controls the adjustment of the stitch regulating device by means of the adjusting device.
It has been known from DE 32 16 993 C3 (U.S. Pat. No. 4,495,877) that the accuracy of positioning of the needle at the end point of the seam can be increased by determining the difference between the desired amount and the actual value of the feed of the feed device by means of a scanning device responding to the passage of the edge of the workpiece and taking this into account in the calculation for setting the stitch regulating device. U.S. Pat. No. 4,495,877 is hereby incorporated by reference. Aside from the fact that such a scanning device, which has at least two sensors arranged at mutually spaced locations from one another, is complicated, there may be inaccuracies during the scanning operation during sewing along curved workpiece edges, e.g., in the case of shirt collars, because of the rotary movement of the workpiece, and these inaccuracies distort the result of the measurement.
It was found that a very essential cause of the observed deviations between the set amount of feed and the actual amount of feed of the workpiece is that the amount of feed of the workpiece per stitch formation operation increases, in general, continuously with increasing speed of the machine, so that the stitch length becomes greater and greater. This feed behavior is taken into account in the sewing machine known from U.S. Pat. No. 4,548,143 by calculating the number of remaining sewing stitches as a function of the machine speed occurring at the time of recognition of the edge. Due to this measure, the speed-dependent changes in stitch length are said not to be able to lead to inaccurate results any more. However, this type of compensation of the speed-dependent changes in stitch length can be used only in processes for moving to a predetermined end point of a seam in which a partial amount of the stitch length is taken into account during the last stitch only.
A sewing machine with a device for measuring the actual feed length of the feed dog has been known from DE 36 17 204 C1 (corresponding to U.S. Pat. No. 4,458,545). Aside from the fact that such a measuring device and the associated signal processing device are comparatively complicated and disturbing effects, e.g., vibrations, may distort the results of the measurement, there is additionally a stitch length deviation which arises from the very principle due to the fact that the measurement of the actual stitch length is carried out at a markedly higher speed and consequently at a greater deviation between the desired value and the actual value of the stitch length than subsequently the formation of the last stitch.
The basic object of the present invention is to provide a sewing machine in which compensation of the speed-dependent changes in stitch length is made possible.
According to the invention, a sewing machine is provided with at least one feed device, with a stitch regulating device, with an adjusting device for the stitch regulating device, with a pulse generator detecting the speed and the angular position of a machine shaft, and with a computer. The computer controls the adjustment of the stitch regulating device by way of the adjusting device. Speed-dependent correction values can be sent to the stitch regulating device.
The present invention is based essentially on the idea of directly counteracting the changes in the effective stitch length arising at higher speeds compared with the set stitch length by a corresponding correction of the setting of the stitch regulating device, so that the changes in stitch length will not appear at all and therefore they do not have to be compensated by measures to be taken subsequently, either. This makes it possible to obtain a more accurate result in all seam sections in which sewing is performed with stitch counting.
In a sewing machine with a device for moving to a predetermined end point of a seam at a spaced location from the edge of a workpiece, a disturbance variable, which would otherwise have an especially strong effect, is eliminated by the compensation according to the present invention of the speed-dependent changes in the stitch length during the sewing of the remaining stitches after the recognition of an edge. Thus, the actual stitch length of the remaining stitches of the seam comes very close to the calculated stitch length, as a result of which a comparatively high accuracy of positioning of the needle at the end point of the seam is achieved.
By taking into account the speed-dependent correction values, the speed-dependent correction values may be stored in the form of a table in a memory and can be read by a computer, to also immediately initiate the proper correction of the stitch length setting already at the beginning of a speed change. Circumventing the measure disclosed in DE 38 04 920 A1, according to which the speed is reduced to a lower value several stitches before the end of the seam, this makes it now possible to continuously reduce the speed to zero in the manner described in DE 196 15 308 C1, and it is guaranteed that the actual length of all remaining whole stitches corresponds to the set stitch length.
The accuracy of a stitch length control and the accuracy of positioning of the needle at the end point of the seam are further increased by determining the value of the generally nonlinear adjustment characteristic of the stitch regulating device and taking it also into account as stitch length-related correction values.